A projection-type display device includes a light source device, display elements that are irradiated by illumination light from the light source device, and a projection lens that enlarges and projects the images displayed on the display elements onto a screen.
Projection-type display devices can be broadly divided between two types.
The first type of projection-type display device is made up of: a white light source; a plurality of dichroic mirrors that split white light into luminous flux of the three primary colors of red (R), green (G), and blue (B); three liquid crystal display elements that are irradiated by respective luminous flux of each of red (R), green (G), and blue (B) and that display images of each color component; a dichroic prism that again synthesizes the luminous flux of three primary colors into a single light beam; and a projection lens. This type of projection-type display device is referred to as a three-panel or 3LCD projection-type display device.
The second type of projection-type display device is made up of: a white light source; a color wheel on which color filters of red (R), green (G), and blue (B) are arranged in a disc form; one display element that, by irradiating light from the white light source onto the color wheel that is rotating at high speed, displays the image of a color component in synchronization with the switching of the color of illumination light in which colors are switched in a time series; and a projection lens. A projection-type display device of this type is referred to as a single-panel, field-sequential, or time-division projection-type display device.
Both types employ a high-luminance light source such as a high-pressure mercury lamp for a white light source. However, a high-pressure mercury lamp, while featuring high luminance, gives rise to the following problems in the projection-type display device that employs such a light source.
Due to the use of mercury, a high-pressure mercury lamp is problematic from the standpoint of the environment, and further, has the problem of short service life.
Because the conditions for stable lighting are pre-determined, dimming to any brightness is not possible. As a result, the brightness of the light source cannot be adjusted according to the conditions of use of the projection-type display device such as the brightness of the room or the magnification of the projected screen, leading to waste of consumed power.
In addition, not only is time required to reach the state of steady brightness after lighting, but after extinguishing the lamp, a waiting period is also necessary for the lamp to cool before immediate relighting, rendering use of the lamp inconvenient.
Recent years have seen the development of higher luminance of light sources referred to as solid-state light sources or semiconductor light sources such as light-emitting diodes (LEDs). A solid-state light source has longer service life than a discharge lamp and is also superior from the standpoint of the environment because mercury is not used.
When an LED is used as the light source in a projection-type display device, power can be saved accurately in accordance with conditions by installing a dimmer function for controlling the amount of current to the LED according to the conditions of use of the projection-type display device.
In addition, a projection-type display device that uses LEDs as a light source obtains a bright image immediately after lighting. Still further, a waiting period for cooling is not necessary before relighting, thereby improving convenience for the user.
Due to the many advantages of solid-state light sources described hereinabove, light source devices that use, for example, LEDs are greatly anticipated in projection-type display devices.
However, in a white LED that emits white light, a fluorescent substance that emits yellow light is excited by blue light to obtain white light by blue and yellow light.
FIG. 1 shows the emission spectrum of a white LED. The emission spectrum has, in addition to a steep peak in the blue wavelength band, a gentle peak in the yellow wavelength band that spreads to green and red. This type of white LED, while having high light-emission efficiency, has a fixed emission spectrum, and the white balance therefore cannot be adjusted. In addition, the chromaticity of white light is known to have variations due to problems arising in the fabrication of LEDs.
In the case of either a three-panel projection-type display device or a single-panel projection-type display device, a color image is basically made up of images of the three primary colors of red (R), green (G), and blue (B). In the case of a white LED, the amounts of green and red light are relatively smaller than for blue or yellow. To obtain a projected image having superior color reproducibility, luminous flux of the narrow wavelength bands of the three primary colors of red (R), green (G), and blue (B) must be extracted from the emitted luminous flux of the white LED. To this end, the luminous flux of yellow must consequently be eliminated, and further, blue luminous flux must be controlled to obtain white balance. In a case in which the color having the least amount of luminous flux is red, the green luminous flux must also be controlled. In other words, the light utilization efficiency is drastically reduced.
When white LEDs are used in interior lighting, a light source having superior color rendering can be obtained by arranging LEDs that emit red and green or blue-green that is insufficient.
In the case of a projection-type display device, however, there is the constraint of etendue that is determined by the angle of divergence and the area of the light source. If the value of the product of the angle of divergence and the area of the light source is not limited to a value no greater than the product of the area of the display element and the acceptance angle (solid angle) that is determined by the f-number of the projection lens, then the light from the light source is not used as projection light. In other words, there is a constraint upon the area of the semiconductor chips of the LEDs or the number of LEDs in the projection optics, and moreover, there is a constraint upon the angle of divergence of the illumination light. An improvement in brightness cannot be achieved even if more LEDs than the number determined by the constraints of etendue are arranged in an array.
In response, Patent Document 1 and Patent Document 2 disclose display devices in which not only brightness and color reproducibility but also white balance are improved by replacing, of the light from a white light source, the light of a specific wavelength band that is insufficient with light from another light source.
According to Patent Document 1, the amount of light of the red wavelength component of a high-pressure mercury lamp that is used as a white light source is small. In response, an LED array light source that emits red light is used in the red illumination light.
According to Patent Document 2, of the white light of a white light source, the light of the red wavelength component for which the amount of light is small is partially replaced by light from a semiconductor laser light source that emits red light using a hologram element.
The above-described prior art are examples in which high-pressure mercury lamps are used as the white light source, but these examples are similar to cases in which the high-pressure mercury lamps are replaced by white LEDs.
In this way, display devices having not only excellent brightness and color reproducibility but also superior white balance, as well, are obtained by using main illumination light and auxiliary illumination light.
As another example in which light of a plurality of colors is synthesized, Patent Document 3 discloses a mode in which the difference in polarization is used to synthesize colored light by a polarization beam splitter or a mode in which the difference in wavelengths is used to synthesize colored light by dichroic mirrors.
According to Patent Document 3, green P-polarized light and red and blue S-polarized light are synthesized by a polarization beam splitter. Alternatively, green light and red and blue light are synthesized by dichroic mirrors.
Synthesizing three different colors from two directions allows the disposition of a greater number of light sources of a color component that is insufficient for emitting white light.